Medical Photonics Lecture 1.2 Optical Engineering · Medical Photonics Lecture 1.2 Optical Engineering. Lecture 10: Instruments III. 2019-06-19. Michael Kempe. Spring term 2019

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Medical Photonics Lecture 1.2Optical Engineering

Lecture 10: Instruments III

2019-06-19

Michael Kempe

Spring term 2019

2

Contents

No Subject Ref Date Detailed Content

1 Introduction Zhong 10.04. Materials, dispersion, ray picture, geometrical approach, paraxial approximation

2 Geometrical optics Zhong 17.04. Ray tracing, matrix approach, aberrations, imaging, Lagrange invariant

3 Diffraction Zhong 24.04. Basic phenomena, wave optics, interference, diffraction calculation, point spread function, transfer function

4 Components Kempe 08.05. Lenses, micro-optics, mirrors, prisms, gratings

5 Optical systems Zhong 15.05. Field, aperture, pupil, magnification, infinity cases, lens makers formula, etendue, vignetting

6 Aberrations Zhong 22.05. Introduction, primary aberrations, miscellaneous7 Image quality Zhong 29.05. Spot, ray aberration curves, PSF and MTF, criteria8 Instruments I Kempe 05.06. Human eye, loupe, eyepieces, photographic lenses, scan lenses

9 Instruments II Kempe 12.06. Microscopic systems, micro objectives, illumination, scanning microscopes, contrasts

10 Instruments III Kempe 19.06. Medical optical systems, endoscopes, ophthalmic devices, surgical microscopes, zoom lenses

11 Photometry Zhong 26.06. Notations, fundamental laws, Lambert source, radiative transfer, photometry of optical systems, color theory

12 Illumination systems Gross 03.07. Light sources, basic systems, quality criteria, nonsequential raytrace

13 Metrology Gross 10.07. Measurement of basic parameters, quality measurements

Limitation of Optical Imaging in Medicine

𝐼𝐼𝐼𝐼0

= 𝑒𝑒𝑒𝑒𝑒𝑒 − 𝜇𝜇𝑠𝑠′ + 𝜇𝜇𝑎𝑎 𝑑𝑑

𝜇𝜇𝑠𝑠′: reduced scattering coefficient(typ. 101 − 102 𝑐𝑐𝑐𝑐−1)

𝜇𝜇𝑠𝑠: scattering coefficient(typ. 102 − 103 𝑐𝑐𝑐𝑐−1)

𝜇𝜇𝑎𝑎: absorption coefficient(e.g.10−1 𝑐𝑐𝑐𝑐−1 @ 1 µ𝑐𝑐 in water)

Penetration - Resolution:

Ballistic light (𝜇𝜇𝑠𝑠) – few mm / several µmDiffuse light (𝜇𝜇𝑠𝑠′) – depth d

Optical Imaging in Medicine

Optical Medical Imaging

Diagnostic Imaging

Ophthalmology

Dermatology

Others

Open SurgeryVisualization

Neuro/Spine

Gynaecology/Urology

ENT

Ophthalmology

Dental

Endoscopy

Gastroenterology

Cardiology

Urology

Pulmonology

Others

Endoscopes: Relay Systems

Endoscopes use various light guiding principles to relay the image overdistance

Rigid endoscopes – slab lens relay Combination of several relay subsystems GRIN lenses or doublet rod lenses and large field-angle objective lens may be used

objective 1. relay 2. relay 3. relay

Ref.: M. Rill

Rigid Endoscopes

0.5

486 nm587 nm656 nm

0.4

00 0.4

Wrms [λ]

0.8 1.2 21.6y'

[mm]

0.3

0.2

0.1diffraction limit

Example: Systems by Storz diameter 3.7 mm

Flexible Endoscopes

Helen D. Ford and Ralph P. Tatam, "Characterization of optical fiber imaging bundles for swept-source optical coherence tomography," Appl. Opt. 50, 627-640 (2011)

Use of fiber bundle array as relay

Each fiber transmits one image point Diameter: typ. 0.5-1.5 mm for 4k to 18k fibers

(data points = pixels) Pixel size: typ. 6-10 µm

Example: System by Storz

Historical Development of Surgical Microscopes

1. Head worn loupe (1876) 4. OPMI (Littmann 1953)2. Corneal loupe (von Zehender/Westien 1887) 5. Contraves Stativ (Yasargil 1972)3. Corneal loupe (Schanz/Czapski 1899)

1.

4.3.

2.

5.

Surgical Microscope

Modern surgical microscopes are stereo systemscombining ocular and digital imaging

Ref.: ZEISS

SurgicalMicroscope

ComputerData TransferPower Supply

Ref.: M. Kaschke et al. Ophthalmology

Zoom Systems

Motivation for zooming: Enlargement of image details Foveated imaging Adaptation of field of view

Basic Principle

Two thin lenses in a certain distance t:Focal length

Refractive power

Many types of zoom systemlayouts

tfffff

−+=

21

21

2121 FFtFFF ⋅−+=

c) Infinite-infinite (I-I)

b) Infinite-finite (I-F)

a) Finite-finite (F-F)

Change of Focal Length

Distance t increased First lens fixed

movedlens

changeddistance

t changed focallength f

Change of Focal Length

Distance t increased Image plane fixed

two lenses moved t f

imageplane

Mechanical Compensated Zoom Systems

Simple explanation of variator and compensator Movement of variator arbitrary Compensator movement

depends on variator:nonlinear

Perfect invariance ofimage plane possible

objectivelens

variatorlinear

compensatornonlinear

relaylens

P

P

P

imageplane

Modular F-F-Setup

Finite-finite configuration wth three parts : 1. Focusing lens2. Zoom group with movable components3. Realy lens

movable zoom lensesfocusing lens relay lensobject image

Symmetrical Three Component I-I Setup

Telescope angle magnification :

Major positions

Symmetrical layout

f1 f1f2

asymmetric 1

Γ > 1

tmax

asymmetric 2

tmin

Γ < 1

symmetric

tm tm

Γ = 1

last

first

hh

ww

=='

Γ

Magnification First distance

Second distance

|Γ| = |Γmax| > 1 tmax 0 |Γ| = 1 tm tm

|Γ| = 1/|Γmax| < 1 0 tmin

Example: Three Groups I-I Optical Compensated

Γmax / Γmin = 6

Γ = 0.41

Γ = 0.92

Γ = 1.41

Γ = 1.92

Γ = 2.44

Mechanical compensation

Variable distances:d3 and d4

Zoom factor 6

18

Optical Ophthalmic Diagnosis

Imaging

Anterior Segment

Slit lamp

OCT

Posterior Segment

Slit lamp

Ophthalmoscope

Fundus Camera

OCT

Measuring

Refractive Power

Objective Refraction:

Autorefractor

Subjective Refraction:Phoropter

Wavefront

Aberrometer

Visual Field

Perimeter

Cornea Topography

Topographer (Placido)

Keratometer

Eye lengths

Biometer (OCT)

Retinal layers

OCT

SL Polarimeter

19

Slit Lamp

Ref.: ZEISS

Köhler (slit) illumination (“slit lamp”)a) from below (Zeiss type)b) from above (Haag Streit type)

Stereo microscope

CMO type Greenough type

http://media.labcompare.com/

20

Slit Lamp

Projection of a slit onto thecornea with small NA

Scattering in the eye Scanning in the anterior of

the eye to detect inhomo-geneities

With the use of (neg.) contact lens or (pos.) auxilliary lens imaging of thefundus is possible

Ref.: M. Kaschke et al. Ophthalmology

Diffuse illumination Slit illumination

ParfocalSwivel

21

Direct Ophthalmoscope

Inspection of an illumination pathreflected on the retina withoutmicroscope

Selection of different aperturesby a rotatable wheel

Compensation lens enforces acoincidence with the observation

Ref.: M. Kaschke et al. Ophthalmology

22

Indirect Ophthalmoscope

Pupil mismatch between patient and observer reduces field of view in directophthalmoscope

Indirect ophthalmoscope: additional ophthalmoscopy lens close to the eye creates an enlarged image of the patients pupil

Ref.: M. Kaschke et al. Ophthalmology

23

Fundus Camera

Observation and photographic inspection of the retina Inspection of the fundus structural analysis to detect morphological deceases Separation of illumination and observation beam path to avoid disturbing reflections Typically ring-shaped illumination

Ref.: M. Kaschke et al. Ophthalmology

24

Fundus Camera Modalities

Ref.: M. Kaschke et al. Ophthalmology

25

Confocal Laser Scanning Ophthalmoscope

Confocal imaging of a fundus spot by scanning (CSLO)

Pinhole mirror separates illumination and detection

Confocal pinhole suppresses straylight

Ref.: M. Kaschke et al. Ophthalmology

Problem: axial optical resolution

∆z ≈ 2λ/NA² ≈ 1.7 µm (1.33 ⋅ 1/24)-2

≈ 555 µm

26

Optical Coherence Tomography (OCT)Using of a low-coherence source enables 3D imaging

Time-domain OCT

Ref.: M. Kaschke et al. Ophthalmology

OCT combines unique resolution with high sensitivity• axial resolution is independent of the numerical aperture (pupil) • by heterodyne detection a photon-noise limited sensitivity is

achieved (R ≥ 10-10) ∆z = lc ≈ 0.44 λ/∆λ≈ 5 µm

27

Optical Coherence Tomography (OCT)

Ref.: M. Kaschke et al. Ophthalmology

Spectral-domain OCT

• Better sensitivity by simultaneous detection of spectral components

• Depth information obtained by Fourier transform

Ref.: ZEISS

28

Optical Coherence Tomography (OCT)

Ref.: Zeiss

OCT-Scan

OCT-Scan

For Glaucoma diagnostics: Either measurement of topology of the blind spotor the thickness of the RNFL

Thickness of RNFL

Measurement against normative database:

Topology of the nerve head

RNFL = Retina Nerve Fiber LayerThe yellow band represents healthy persons

is measured by a circular OCT scan

Depth information enables measurement of layer thickness for diagnosis

29

Refractometer

Autorefraction measurement of the eye power

Test pattern projected onto the retina (mire = target pattern)

Fundus reflected light is observed (Ophthalmoscope)

z-differences correspond to focal power errors (optometer method)

Ref.: M. Kaschke et al. Ophthalmology

30

Aberrometer

Measurement of the human eye wavefront with a Hartmann-Shack wavefront sensor

Illumination spot on the fundus reflected

Ref.: M. Kaschke et al. Ophthalmology

31

Keratometer

Measuring the refractivepower of the cornea

Main contribution: curvature,only R measured

Principle:Determination of image size y‘

To correct for motion a double image is used as reference

Ref.: M. Kaschke et al. Ophthalmology

1𝑠𝑠′

=1𝑠𝑠

+1𝑓𝑓′

𝑦𝑦′𝑦𝑦

=𝑠𝑠′𝑠𝑠

𝑟𝑟𝑐𝑐 = 2𝑓𝑓′ = 2𝑠𝑠 � 𝑦𝑦′𝑦𝑦

y’∆y

32

Keratometer

Helmholtz-type keratometer

Littmann keratometer

Achieved accuracy: ∆rc = 0.05...0.1 mm

Ref.: M. Kaschke et al. Ophthalmology

33

Cornea Topography by Placido Disk

Projection of a ring mask onto the cornea (Placido mask) Imaging the rings onto a camera Evaluation of the imaged ring widths Reconstruction of the topology of the cornea

Ref.: M. Kaschke et al. Ophthalmology

projected patternimage

reconstructedtopology

real deformedimage

34

Corneal Topographer

Realization of the Placido-projectionand imaging of the reflected light

Ring-by-ring reconstruction of thecornea surface

Ref.: M. Kaschke et al. Ophthalmology

35

Biometer (OCT)

Ref.: ZEISS

Special OCT technique to determine:

Measurement of full eye along optical axis possible with time-domain OCT (double beam technique) or swept source OCT

Low lateral resolution ensures large depth of focus